Answer
$4.2$
Work Step by Step
RECALL:
(i) Scientific notation has the form $a(10^n)$, where $1 \le a \lt 10$ and where $n$ is an integer.
(ii) $a^m \cdot a^n = a^{m+n}$
(iii) $a^0=1, a\ne 0$
Write each factor in scientific notation to obtain:
$=2.1(10^{-3}) \cdot 2(10^3)$
Multiply $2.1$ and $2$ together, and then multiply the powers of 10 together to obtain:
$=(2.1 \cdot 2)(10^{-3} \cdot 10^{3})
\\=4.2(10^{-3+3})
\\=4.2(10^{0})
\\=4.2(1)
\\=4.2$
